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Structure of Sodium Carboxymethyl Cellulose Aqueous Solutions: a SANS and Rheology Study

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Structure of Sodium Carboxymethyl Cellulose Aqueous Solutions: a SANS and Rheology Study JOURNAL OF FULL PAPER WWW.POLYMERPHYSICS.ORG POLYMER SCIENCE Structure of Sodium Carboxymethyl Cellulose Aqueous Solutions: A SANS and Rheology Study Carlos G. Lopez,1 Sarah E. Rogers,2 Ralph H. Colby,3 Peter Graham,4 Joao~ T. Cabral1 1Department of Chemical Engineering, Imperial College London, London SW7 2AZ, United Kingdom 2Diffraction and Materials Division, ISIS-STFC, Rutherford Appleton Laboratory, Chilton, Oxon OX11 0QX, United Kingdom 3Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802 4Unilever Research Port Sunlight Laboratory, Quarry Road East, Bebington L63 3JW, United Kingdom Correspondence to: J. T. Cabral (E-mail: [email protected]) Received 11 August 2014; revised 2 December 2014; accepted 5 December 2014; published online 30 December 2014 DOI: 10.1002/polb.23657 ABSTRACT: We report a small angle neutron scattering (SANS) ing semidilute unentangled and entangled regimes, followed and rheology study of cellulose derivative polyelectrolyte by what appears to be a crossover to neutral polymer concen- sodium carboxymethyl cellulose with a degree of substitution tration dependence of viscosity at high concentrations. Yet of 1.2. Using SANS, we establish that this polymer is molecu- those higher concentrations, in the concentrated regime larly dissolved in water with a locally stiff conformation with a defined by rheology, still exhibit a peak in the scattering func- stretching parameter B ’ 1:06. We determine the cross sec- tion that indicates a correlation length that continues to scale 21=2 tional radius of the chain (rp ’ 3.4 A˚ ) and the scaling of as c . VC 2014 The Authors. Journal of Polymer Science Part the correlation length with concentration (n 5 296 c21=2A˚ for c B: Polymer Physics Published by Wiley Periodicals, Inc. in g/L) is found to remain unchanged from the semidilute to J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 492–501 concentrated crossover as identified by rheology. Viscosity measurements are found to be in qualitative agreement with KEYWORDS: polyelectrolytes; cellulose; neutron scattering; rhe- scaling theory predictions for flexible polyelectrolytes exhibit- ology; structural characterization; water-soluble polymers 7 INTRODUCTION tile treatments and to prevent the redeposition of soil removed by detergents during the fabric washing process.8 Its Cellulose and Cellulose Derivatives monomer structure is shown in Figure 1, where R stands for Cellulose is the most abundant polymer on Earth with annual AHorACH CO Na. The degree of substitution (D.S.) is the cellulose synthesis by plants approaching 1012 tons.1 Although 2 2 average number of carboxymethyl groups substituted per cellulose is insoluble in water and most common organic sol- monomer unit, ranging from 0 to 3 with the remaining R 5 H. vents,2 cellulose derivatives are soluble in a wide range of sol- 3 vents; for example, cellulose acetate is soluble in acetone, Aqueous solutions of NaCMC are generally complex. Some stud- tetrahydrofuran, and other organic solvents while methylcellu- 9,10 ies suggest that only a fraction is molecularly dispersed, lose, ethyl hydroxyethyl cellulose, and sodium carboxymethyl implying that NaCMC is a colloidally dispersed polymer while cellulose (NaCMC) are water soluble.2,4 Cellulose derivatives others report full solubility in dilute salt solutions.11,12 The can be dissolved either at a molecular or colloidal level.2,3 degree of solubility is accepted to be a function of the degree of Carboxymethylcellulose (CMC), generally used as sodium salt substitution, with less substituted (more hydrophobic) NaCMC 10 NaCMC, is one of the most widely used polyelectrolyte cellu- showing a larger fraction of aggregates. Aggregated crystalline lose derivatives.5 NaCMC is an anionic, water soluble, polyelec- domains have been found by X-ray diffraction in aqueous solu- 13 trolyte with vast applications in the food, pharmaceutical, tion, with degree of crystallinity correlating with lower D.S. and personal care/cosmetic, paper and other industries.4 For no crystallinity found for D.S. 1.06. Additionally, low substitu- instance, NaCMC is extensively used as a rheology modifier in tion grades often exhibit thixotropy10,14 which is thought to common toothpastes and shampoos,6 as a film former in tex- result from the formation of networks or soft aggregates.10 The Additional Supporting Information may be found in the online version of this article. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. VC 2014 The Authors. Journal of Polymer Science Part B: Polymer Physics Published by Wiley Periodicals, Inc. 492 JOURNAL OF POLYMER SCIENCE, PART B: POLYMER PHYSICS 2015, 53, 492–501 JOURNAL OF POLYMER SCIENCE WWW.POLYMERPHYSICS.ORG FULL PAPER BACKGROUND THEORY Scaling Theory e2 The Bjerrum length lB5 , where e is the unit of charge, 4p0kBT 0 is the vacuum permitivity, is the relative dielectric con- stant, and kB is the Boltzmann constant, is defined as the distance at which the electrostatic energy between two FIGURE 1 Monomer structure of sodium NaCMC, where R can charges is equal to the thermal energy (7.1 Å for water at be H or CH COONa. The degree of substitution (D.S.) is defined 2 25 C). Salt free solutions of flexible polyelectrolytes can be as the average number of CH COONa groups per monomer 2 classified into dilute, semidilute (non entangled and out of a maximum of 3. [Color figure can be viewed in the entangled), and concentrated. In dilute solutions, c < c*, online issue, which is available at wileyonlinelibrary.com.] where c* is the overlap concentration, polyelectrolytes adopt an extended conformation and can be represented by a directed random walk of electrostatic blobs of diameter nT. regularity of monomer substitution, instead of the average D.S., ( 2 1=3 was found to be the main factor controlling the rheological prop- bðA b=lBÞ ; T h erties of NaCMC aqueous solutions.10 NaCMC of low D.S. but uni- nT 5 (1) b A2b=l 3=7; T h form substitution, that is, lacking blocks of unsubstituted ð BÞ monomers, yields similar properties to highly substituted grades. where A is the number of monomers per effectively charged Typically, samples with D.S. տ 1, do not exhibit thixotropy as, group, h is the theta temperature, and b is the monomer length. with increasing D.S., substitution becomes more random. For this For distances below nT, thermal energy dominates and the chain study, we have selected a sample of D.S. ’ 1.2 and, therefore, conformation depends on the solvent quality, becoming extended expect no significant blocks of unsubstituted cellulose, and thus in good solvent and collapsed in poor solvent. The stretching no thixotropy or gelling at high concentrations. According to a parameter (B5bgT =nT ), where gT is the number of monomers in 15,16 model by Reuben et al., for an average D.S. 5 1.2, 10–20% of an electrostatic blob, is defined as the ratio between the polymer the monomers will not be substituted and thus, even in the contour length L 5 Nb,whereN isthedegreeofpolymerization, absence of large unsubstituted blocks, hydrophobic interactions and the end-to-end length in dilute solution, Ree 5 NnT =gT .The from unsubstituted monomers remain. A range of intrinsic per- scaling prediction for the specific viscosity at zero shear rate sistence lengths (l ) for NaCMC have been reported, varying g02gs 0 (gsp g ,whereg0 is the dynamic viscosity at zero shear rate 11,17,18 s from 5 to 15 nm characteristic of a semiflexible polymer and gs is the solvent’s dynamic viscosity) in dilute solution is gsp 18,19 à and typical of many polysaccharides. c=c . Throughout this article, we use gsp to refer to the specific viscosity in the Newtonian (zero shear rate) limit. The rheology of NaCMC in aqueous, salt free, solutions has been previously studied.5,10,14,20–24 A Fuoss or near Fuoss depend- At c*, estimated by 1=2 ence of the specific viscosity (gsp / c ) has been reported in 20–22,24 the semidilute unentangled regime. Astrongdepend- N cà ’ 5B3b23N22 (2) ence of the specific viscosity (gsp) with polymer concentration R3 3:7525:5 20,24 ee (gsp / c ) is reported at high concentration. chains begin to overlap. In the semidilute regime, the most rele- 25 Simulations by Wang and Somasundaran suggest that vant length scale is now the correlation length n;atlength NaCMC adopts a helical conformation in bulk and in solution scales smaller than n, chains do not interact and the conforma- 26 while Biermann et al. found that, depending on the tion is like that of a dilute solution; at length scales larger than substitution pattern, the chain may be locally collapsed or n, the electrostatic energy is screened and chains follow random highly extended. Although a helical chain conformation is coil statistics. A peak in the structure factor, S(q), is observed in suggested by optical rotary dispersion and circular flexible semidilute salt free polyelectrolytes, whose position 27–29 dichroism data, light scattering, intrinsic viscosity, scales as qà / c1=2, in agreement with the scaling prediction: potentiometric titration, and dielectric spectroscopy data have been successfully interpreted assuming a locally linear n5ðB=cbÞ1=2 (3) conformation.11,17,24,30,31 Wavenumber qà can be related to the correlation length, n, Significant questions regarding the local conformation and by qÃ52p=n. The origin of this peak has been assigned to solubility of NaCMC in water thus remain unresolved. In this the inability of correlation blobs to overlap due to the strong work, we use small angle neutron scattering (SANS) to probe repulsion generated by the counterion clouds surrounding the solubility and conformation of NaCMC in aqueous solu- the polyions, thereby resulting in the suppression of long tion.
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